The present invention relates to a steering control apparatus that controls the attitude of a vehicle and method for steering control.
A road surface having different friction coefficients in left and right sections is referred to as a surface with split coefficient of friction, that is, a μ-split road surface. When hard braking is applied to a vehicle on such a μ-split road surface, and antiskid control is executed, the generated braking force is different in the left and right sides of the vehicle. Due to the difference of the braking force in the left and right sides, that is, left-and-right braking force difference, yaw moment is generated which deflects the vehicle toward the side with a higher friction coefficient. On a typical μ-split road surface of an actual road, for example, the friction coefficient of the road surface is lowered due to snow and ice remaining on the road shoulder, and is increased in a central section of the road due to dry or wet asphalt. If a vehicle driving on such a surface with left and right wheels on the road shoulder and the central section, respectively, is abruptly stopped, the braking force is greater in the central section, where the friction coefficient is high, than in the road shoulder, where the friction coefficient is low. As a result, yaw moment is generated in the vehicle, which deflects the vehicle toward the central section of the road.
To suppress such a yaw moment generated due to a left-and-right braking force difference, a driver needs to steer in a direction opposite to the direction in which the vehicle is deflected. Such control is known as countersteering. However, countersteering requires a certain level of skill.
Japanese Laid-Open Patent Publication No. 2001-334947 discloses a system that suppresses yaw moment generated due to a left-and-right braking force difference. Specifically, the system, based on the difference between rotation speeds of the left and right wheels, controls a steering mechanism to add a controlled steered angle to the wheel with the smaller rotation speed.
However, since the system according said publication causes difference in rotation speeds of wheels and generated moment to vary due to the antiskid control, it is sometimes difficult to effectively stabilize the vehicle attitude.